Biomecanica

The Biomechanics of Femoroacetabular Impingement

Daniel E. Martin, MD, and Scott Tashman, PhD

Femoroacetabular impingement (FAI) is proposed as a possible biomechanical etiology of

early, idiopathic hip osteoarthritis (OA). Two primary mechanisms are proposed: cam

impingement and pincer impingement. In cam impingement, an abnormally shaped or

excessively large femoral head or neck abuts against the anterosuperior acetabulum. In

pincer impingement, overcoverage of the proximal femur by the acetabulum results in

impingement. In severe cases, a contre-coup mechanism results in an anterosuperior

contact point that functions as a fulcrum and produces posteroinferior impingement as the

femoral head is levered out of the acetabulum. However, these proposed mechanisms are

made on the basis of surgical observation rather than in vivo documentation of FAI, and

controversy exists as to whether surgical interventions should be made on the basis of

these theories alone. In this review of FAI biomechanics we discuss the proposed biome-

chanical mechanisms of FAI, the analytical methods currently available to study FAI

biomechanics, and the topics that future biomechanical studies of FAI will need to address.

Ultimately, a better understanding the biomechanics of FAI may help physicians design

interventions that decrease the risk of progression to hip OA.

Oper Tech Orthop 20:248-254 © 2010 Elsevier Inc. All rights reserved.

KEYWORDS

cam impingement, femoroacetabular impingement, hip biomechanics, pincer im-

pingement

F

emoroacetabular impingement (FAI) occurs when the

head or neck, or overcoverage of the proximal femur by

head or neck of the femur abuts against the rim of the

the acetabulum.

acetabulum. The principles of hip impingement are studied

Although these anatomic features can be easily recognized

with regard to total hip arthroplasty, in which components

by the use of readily available imaging techniques, such as

must be designed to minimize wear and dislocation.

Im-

plain radiographs, in vivo characterization of abnormal con-

1 -3

tact between the femur and the acetabulum proves more

difficult. Devising and implementing appropriate surgical in-

pediatric hip disorders, where dysmorphic native anatomy or

terventions, therefore, is also difficult. In this review we aim

surgically altered anatomy provides a readily identifiable

to summarize the proposed biomechanical mechanisms of

source of impingement.

4-7

The recognition of hip impingement in these patient pop-

FAI, the analytical methods currently available to study FAI

ulations has led several authors to examine FAI as a potential

biomechanics, and the topics that future biomechanical stud-

ies of FAI will need to address.

cause of early, idiopathic osteoarthritis (OA) in younger pa-

tients. The work of Ganz et al is particularly instrumental in

defining FAI, as this group has performed surgical disloca-

Proposed Mechanisms of FAI

tion of the hip in several hundred patients with symptomatic

impingement and has meticulously documented their in-

Ganz et al

proposed FAI as a mechanism for the develop-

9

traoperative observations.

These observations provide

8 -1 0

ment of early OA in the absence of dysplasia after performing

the basis for 2 proposed mechanisms of FAI: an abnor-

surgical dislocation of the hip on more than 600 symptomatic

mally shaped (nonspherical) or excessively large femoral

patients. On the basis of the location of labral and articular

cartilage pathology, the authors suggested that FAI occurred

most often in terminal exion and that additional shearing

University of Pittsburgh School of Medicine, Department of Orthopaedic

damage could occur if terminal exion was accompanied by

Surgery, Pittsburgh, PA.

rotation. Furthermore, the authors suggested that the im-

Address reprint requests to Scott Tashman, PhD, University of Pittsburgh

pingement could result from 2 possible morphologic abnor-

Biodynamics Laboratory, 3820 South Water Street, Pittsburgh, PA

malities, the cam lesion and the pincer lesion.

15203. E-mail:

tashman@pitt.edu

248

1048-6666/10/$-see front matter © 2010 Elsevier Inc. All rights reserved.

doi:10.1053/j.oto.2010.09.015

Biomechanics of FAI

249

Defining the Normal Hip

radius of the femoral head and that of the femoral neck,

which allows a greater range of motion about the hip (

Fig.

In describing biomechanical abnormalities, it is important to

2A, 2

B). The alpha angle is proposed to evaluate deviations in

understand the criteria by which normal hip morphology is

the sphericity of the femoral head and the normal offset be-

generally described, which is drawn largely from the study of

tween the femoral head and the femoral neck.

The alpha

hip dysplasia.

Because it best identifies labral and cartilage

14

11

angle is measured between a line parallel to the axis of the

pathology, magnetic resonance arthrography is the gold stan-

femoral neck and a line drawn from the center of the femoral

dard in clinical imaging.

However, the following measures

12

head to the point at which the distance from the center of the

focus on the bony abnormalities presumed to cause FAI.

femoral head to the cortex of the femoral head or neck first

The center-edge angle (CEA) was developed to quantify

exceeds the radius of a circle fit to the femoral head (

Fig. 1

B).

hip dysplasia in which the acetabulum is too shallow, thus

Although the values that indicate pathology are debated, val-

predisposing patients to instability of the hip joint. The CEA

ues 50° are generally accepted to represent normal proxi-

is measured on an anteroposterior (AP) radiograph of the hip

mal femur morphology.

as the angle between avertical line that intersects the center of

Gosvig et al

also recently proposed the triangular index

the femoral head and a line that is drawn from the center of

15

for evaluation of proximal femoral morphology. The triangu-

...